In the fabrication of safety glass for automotive windshields and the like, it is known to sandwich a thermoplastic layer, hereinafter referred to as a plastic sheet, between two singly-curved or doubly-curved glass panes, and to bond the resulting stack into a composite or laminate.
The reference to single curvature here is to curvature in one plane and a reference to double curvature is a reference to curvature in two mutually-perpendicular planes.
The laminate can have, therefore, a glass outer pane, a glass inner pane and a plastic sheet between the two panes forming an interlayer.
It is common to provide a coating by a screen-printing process, e.g. to the surface of the outer pane facing the plastic sheet, of a conductive or nonconductive material. For example, when an edge pattern of the printing is provided, the edge coating can be used to obscure other components of the motor vehicle. It can also take the form of a printed antenna, if it is conductive, or can be a heating element operating by resistance heating.
The coating, regardless of its purpose and utility, has exacting requirements. It must satisfactorily survive the statistical service life of the laminate and neither discolor nor detach. The coating must be clearly defined, i.e. the printed borders must be maintained and not run. The coating must provide a uniform covering, free from blisters and gas bubbles or inclusions and provide an overall favorable appearance.
Printed coatings of this type, composed of enamel inks and applied by screen printing, have in the past met many of these requirements, but almost invariably have required an elaborate process for producing a single or double curvature.
The production of a laminate with single or double curvature may use the following known processes. Two panes which are to form the inner and outer panes of the laminate are placed one on top of the other and heated to a deformation temperature to bend the pair of plates to the single or double curvature as required. The curved panes are then removed from one another, the plastic sheet placed between them and the stack with the plastic sheet interposed is heated to form the laminate.
This process is, by and large, satisfactory and, since plates conform in configuration to one another, they have the same curvature and, apart from negligible tolerances, are largely free from optical distortion. In the past, however, when a screen-printing process was used to apply a standard enamel ink, this approach could not be employed.
The enamel ink, which could include ethyl cellulose as an organic vehicle, was applied to the flat uncurved glass pane and baked thereon at a temperature of say 555.degree. C.
The two panes, both of which have baked on coatings, can then be placed one upon the other and heated to deformation temperature to impart the single or double curvature thereto. The process then continued as described above, i.e. the panes were separated, the plastic sheet interposed and the stack heated to form the laminate.
In this process, however, the baking step interfered with the process line for producing the laminate and involved a costly procedure which was not readily integratable into the standard production line for producing laminates. The baking process moreover, when carried out with the planar glass panes, introduced deformation with optical faults. These optical faults remained pairing of the glass panes and bending to single or double curvature.
When efforts were made to eliminate the prebaking, the adhesion of the enamel ink was found to be inadequate and blistering of the enamel coating was observed.